1. Field of the Invention
The present invention relates to a resistor string circuit for use in a high-precision D/A converter or an A/D converter.
2. Description of the Related Art
An example of a resistor string circuit of this type is shown in "Non-Linear Error Analysis And Evaluation of Resistor String Type A/D Converter", Treatise Collection of the Electrical Communication Society of Japan, vol. J67-C, No. 12, PP 941-948, 1984-12.
A resistor string type D/A converter is generally used as an internal circuit of a D/A converter device or an A/D converter device. The precision of the resistor string type D/A converter is dependent on the manufacturing errors of the resistor elements. In other words, if the resistance of one resistor element differs from that of another due to the manufacturing errors, the resistor string produces a non-linear error. This being so, the precision of a conventional resistor string type D/A converter is not more than 8-9 bits, and it is difficult to realize a high-precision D/A converter or A/D converter of 10 bits or more by using the conventional resistor string type D/A converter.
FIG. 1 is an equivalent circuit diagram showing an example of a 3-bit resistor string circuit disclosed in the document noted above. As is shown in FIG. 1, resistor elements R1 to R8 are connected in series between a ground potential point Vss and a reference potential point Vref.
FIG. 2 shows an example of a circuit pattern corresponding to the resistor string circuit of FIG. 1. In the resistor segment circuit shown in FIG. 2, resistor elements R1 to R8 are arranged in such a manner as to extend in the Y-direction, to thereby shorten the X-direction dimension of the circuit pattern. Resistor elements R1 to R8 are connected together by means of metal wiring layers (e.g., aluminum wiring layers Al) extending in the X-direction.
FIG. 3 is a graph showing how a deviation of a planar component per X-direction unit length is produced in the circuit pattern shown in FIG. 2 due to the manufacturing error. In the graph of FIG. 3, the position of a resistor is plotted against the abscissa, while the resistance .rho.(.OMEGA./.mu.m) per unit length is plotted against the ordinate.
FIG. 4 is a graph showing how a non-linear error .epsilon. occurs in the case where the circuit pattern in FIG. 2 has such X-direction resistance deviations as are depicted in FIG. 3 (the deviation of a planar component in the X-direction is neglected in FIG. 4). In FIG. 4, the potential divided by the resistor elements is plotted against the abscissa, while the non-linear error .epsilon. is plotted against the ordinate. In the case shown in FIG. 4, the maximum error is .epsilon.0.
FIGS. 5 and 6 show another conventional resistor string circuit. FIG. 5 is an equivalent circuit diagram, and FIG. 6 is a plan view of the circuit pattern.
The resistor string circuit shown in FIGS. 5 and 6 is turned back at the portion between resistor elements R4 and R5, and a ground potential point Vss and a reference potential point Vref are located adjacent to each other.
In the case of the circuit pattern shown in FIGS. 5 and 6, the maximum value .epsilon.1 of the non-linear error is lower than that .epsilon.0 of the case shown in FIGS. 1 and 2 (.epsilon.1&lt;.epsilon.0), as is shown in FIG. 8, although the deviation of a planar resistance component per X-direction unit length remains the same, as is shown in FIG. 7. However, maximum value .epsilon.1 is not more than about 1/2 of maximum value .epsilon.0. Therefore, even where a D/A converter including the resistor string circuit shown in FIGS. 5 and 6 is employed, it is difficult to realize a high-precision D/A or A/D converter of 10 bits or more.
As described above, with the conventional resistor string circuits, the non-linear error arising from the manufacturing error cannot be reduced to a satisfactory extent, and it is difficult to realize a high-precision D/A or A/D converter device by employing a D/A converter including either one of the above conventional resistor string circuits.